In electophotography, there is a common need for inexpensive, easily fabricated, resistive polymeric matrix compositions, such as films or resins, etc., such as for use in electrical contacts, electromechanical contacts, electrostatic contacts, and devices, which vary over a substantial resistance range. The resistance of the films as well as the surfaces they provide can be changed by varying the quantity of conductive material dispersed in an insulating binder. A greater resistance can be achieved by lower loadings of the selected conductive material, where small decreases at the percolation threshold in loading of conductive materials can cause dramatic increases in resistance. Typically, such materials have a surface resistivity in the range from about 102 ohms/square to about 108 ohms/square and a thickness in the range from about 10 nanometers to about 1 millimeter. For example, thin films having a resistivity targeted at a desired value within such ranges can be used to overcoat other materials to comprise a multiple-layer component. As a result, the surface layer of such a coated component can exhibit for example static discharge, electrostatic bleed-off behaviors, current conduction, resistive heating, and other similar characteristics. However, it can be difficult to precisely control and maintain films or resin based composites associated with known resistivity values or resistivity ranges due to the occurrence of sudden resistance changes that can be caused by improper selection of material compositions used to make the subject films or resin composites and which occur at, or near specific percolation thresholds which are known to represent a particularly sensitive region of the resistivity-filler loading spectrum. Dramatic increases or even decreases in resistance can be observed when conductive particles or fillers are incorporated into such composite materials, which render material composites conductive and then become subjected to external or internal forces that cause a change in the initial relationship, such as particle-to-particle distance or effective fill density that exists between the conductive filler and host. The host can be a polymeric resin such as a plastic or elastomer, a ceramic or glass, a metal, or combinations thereof. An example of an external force that can cause an effective change in the resistivity of a filled composite is a compressive force of such magnitude to cause significant compression or density change in the composition. Thermal or humidity induced swelling can also cause such instabilities.
Conductive particles have been loaded in composites in varying quantities to control resistance levels. For example, light loadings of conductive particles, for example <30% by weight, have been added to insulating host matrices, such as polymers in attempts to achieve a target resistivity value. Naturally, it is desirable to eliminate dramatic changes in resistance that can occur over the functional life of the related device, which can be further complicated when the target resistance value falls at, or close to a percolation threshold. In addition, the ability to precisely control all of the material properties of such a composite can be hampered by inhomogeneities that result from poor dispersion of small size fillers and low material amounts to a host matrix polymer. To reduce this effect, filler materials that are relatively less conductive have been used at relatively high loadings. For example, various metal, metal oxide containing particles, and carbon black particles with volume resistivities selected to represent the higher end of the available resistivity range have been used in attempts at achieving good solid-stage dispersion and tightly controlled electrical resistivities. However, high loadings of particles in a thin film can cause other unwanted effects, for example they are known to make the film hard or brittle or can cause low toughness and tear strength properties.
An example of the need for resistive compositions with controlled electrical properties can be found in corona charging devices, such as scorotrons. However, the device suffers from a number of problems. Any differences in the microstructure of the pins causes each pin to form a corona at a slightly different voltage. Once a corona forms at the end of a pin, the voltage on the array of pins drops, because the corona sustaining voltage is less than the corona onset voltage. The drop in voltage prevents other pins from forming a corona. This self-limiting behavior can be overcome by including current-limiting resistances between each pin and the bus bar which supplies the high voltage to all of the pins in the array. However, it is difficult to control the individual distributed resistances between the pins and bus, because the required resistivity for such devices is generally at the edge of the percolation threshold for most materials. Any small, local changes in composition result in large changes in resistivities making it difficult to obtain a precisely controlled and uniform resistivity across all of the thin film resistors that are in a large population.
A general example of the need for resistive matrix compositions having tightly controlled resistivity values can be found in simple voltage sensors for electrostatically charged surfaces. A high voltage sensor fabricated with a resistive film having a desired target circuit resistance bleeds only a small quantity of charge from a surface leaving the charge density nearly unchanged. The need for the disclosed resistive compositions can also be found in document sensing devices in xerographic copying machines. As a document or paper passes between an electrical contacting brush and a resistive film, the resistance of the circuit is changed.
In general, desired resistivity of a conductive composition can be achieved by controlling the type, shape, and loading of the conductive particles and/or other filler materials. Very small changes in the loading of conductive filler materials near a threshold value at which bulk conduction occurs, i.e., the percolation threshold, can cause dramatic and unwanted changes in a composition's conductivity. Furthermore, differences or variations in particle chemical composition, form, size and shape can cause variations in conductivity at even a constant weight loading. Moreover, the relative change in resistivity with filler loadings is generally less with loadings substantially above the percolation threshold. However this generally requires sufficiently high concentrations of conductive particles, in order to assure conductive particle-to-particle contacts to effectively span the thickness of the composite. The percolation threshold is effectively achieved at the point where a first continuous particle chain is formed and results in an extremely large change in conductivity with respect to incremental changes in filler loading. Clearly, in the case where there is only one continuous chain that establishes the threshold, any change to the continuity of this chain will have a dramatic effect on the resultant conductivity In order to assure that a sufficient number of chains exist and in order to assure that the subject composition has a generally stable electrical resistivity, often a larger than necessary fill loading is employed in the composition. As a result the relative cost of the filler, which is often more expensive than the host matrix material, can dominate the overall cost of the composite. Generally lower filler loadings are desired from an economic perspective.
In general, the current-voltage response of a particle-filled composite is an important design consideration for electric circuits and related devices that employ such composites. A linear current-voltage response is known in the art as “ohmic” or also described as obeying Ohms law. Similarly, non-linear current-voltage responses are referred to as “non-ohmic”. It is known that many conductive particle filled polymer composites, for example carbon black filled plastics, behave non-ohmically when subjected to a variable applied voltage. Since many commercial devices are subjected to operational situations that require variable applied voltages, often varying by hundreds or even thousands of volts, the non-linear response is an undesired characteristic that complicates the device design and adds unnecessary design and product costs.
As conventionally known in the art, conductive filler materials generally have DC volume resistivity values from less than about 10−3 to about 10−6 ohm-cm, while insulating materials, on the other hand, generally have resistivity values of greater than about 1013 ohm-cm to about 1016 ohm-cm. “Controlled conductivity” materials having intermediate resistivities can have resistivity values ranging from about 10−3 ohm-cm to about 1013 ohm-cm.